The conventional dialing disk is being gradually displaced by keyboards with pushbuttons whose operation causes the generation of predetermined frequencies associated with respective digits, usually pairs of frequencies selected from two groups of four frequencies each. The installation of such multifrequency generators is advantageous for the subscriber in that it speeds up and simplifies the transmission of call digits; problems arise, however, where keyboard-equipped stations coexist with dial-equipped stations in a group of lines terminating at a common exchange whose selector stages are necessarily designed to handle digital pulses generated by the dialing disk.